1. Field of the Invention
This invention relates to an improved method of separating carbon dioxide from fluids, and more specifically, this invention relates to an improved method of separating carbon dioxide from flue gases which utilizes immobilized amine sorbents having increased stability under high humidity and high temperature conditions such as those found in flue gases. The same method can also be used for the removal of carbon dioxide from ambient air.
2. Background of the Invention
Carbon dioxide capture is the separation of CO2 from emissions sources or from the atmosphere with the subsequent recovery of a concentrated stream of CO2 that is amenable to sequestration or conversion. Pulverized coal (PC) plants, which are 99 percent of all coal-fired power plants in the United States, burn coal in the presence of air to create steam. Carbon dioxide is exhausted as part of the flue gas at atmospheric pressure and a concentration of 10-15 volume percent of the flue gas. The post-combustion capture of CO2 is a challenging application because (1) the low pressure and dilute concentration of the carbon dioxide dictate a high actual volume of gas to be treated; (2) trace impurities in the flue gas tend to reduce the effectiveness of the CO2 absorbing processes; and (3) compressing captured CO2 from atmospheric pressure to pipeline pressure (1,200-2,000 pounds per square inch (psi)) represents a large parasitic load.
Given the increased global warming due to the presence and production of greenhouse gases such as carbon dioxide (CO2), the capture and permanent sequestration of carbon dioxide by economical means becomes an imperative mission. The “benchmark” process for post-combustion CO2 capture at coal-fired plants is the commercially available monoethanolamine (2-aminoethanol) (NH2CH2CH2OH) (MEA) wet scrubbing system. In this process, an aqueous MEA solution contacts the flue gas in an absorber to absorb the CO2. The subsequent solution containing absorbed CO2 is then heated in a regenerator to release CO2. This system is an example of a temperature-swing absorption (TSA) process. Since the concentration of MEA is limited to 30% due to corrosion issues, large quantities of water must be handled and heated during operation. The limited CO2 transfer capability of MEA results in high regeneration rates for adequate CO2 loading and regeneration. All together, these characteristics make MEA-based CO2 capture systems very energy intensive.
Direct capture of CO2 from the air is an alternate technology to the capture of CO2 from large point sources, and this has the advantage that it can address CO2 emissions from all sources if the technology is operated on a sufficiently large scale. However, this presents the technical challenge to develop adsorbents that operate near ambient conditions and that can extract CO2 from ultra-dilute sources with CO2 concentrations ranging from 200 to 600 parts per million (ppm).
Large-scale deployment of carbon capture from point sources, coupled with carbon sequestration, can be accomplished through several different pathways which include cryogenic distillation, membrane purification, absorption with liquids, and adsorption using solids.
For the capture of CO2 from both flue gases and ambient air, it is near critical that CO2 sorbents must be designed to be reused extensively in a commercial CO2 capture process, maintaining a high cyclic stability under realistic operating conditions.
Polyethyleneimine (PEI) has been used as a CO2 sorbent while the PEI itself was adsorbed to inert silica substrates. This sorbent has displayed instability under steam-stripping conditions to remove adsorbed carbon dioxide. This instability may be due to the lack of covalent bonds between the aminopolymer and the inert silica support, and also due to the measurable water-solubility of low-molecular-weight PEI.
It is possible to synthesize thermally stable solid CO2 sorbents with significantly reduced water solubility so the typical conditions of steam regeneration used to release the adsorbed CO2 will not degrade the sorbents. The syntheses which are presently possible require coupling the amine sorbent with another moiety. These syntheses are multistep and expensive. The key to the commercialization of such a sorbent is the ability to manufacture the sorbent in bulk with minimal handling and simple equipment.
At present, there is not a method which can make amine sorbents wherein the amine is simultaneously (via one-step) coupled with another moiety and physically and chemically bonded to a substrate making the coupled amine sorbents very stable under high humidity multicycle use which includes heating to release CO2.
A method is needed to stabilize solid amine sorbents under high humidity multicycle conditions which include heating. A need also exists for an improved method to produce physically and chemical stable amine sorbents for CO2 so as to reduce costs due to spoilage of the amine sorbents. Yet another need exists for an amine sorbent which can adsorb CO2 under a range of conditions from ambient atmospheres to point sources. Still another need exists for amine sorbents which demonstrate high CO2 capacities under real CO2 capture conditions. Yet another need exists for a one-step scalable synthesis of a coupled amine sorbent, suitable for economically viable production of commercial quantities.